I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for transmitting variable rate data in a wireless communication system, and for assisting a hard handoff.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation techniques of CDMA have significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, assigned to the assignee of the present invention, and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled xe2x80x9cMETHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, and U.S. Pat. No. 5,109,390 entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both assigned to the assignee of the present invention and incorporated by reference herein.
A method for transmission of speech in digital communication systems that offers particular advantages in increasing capacity while maintaining high quality of perceived speech is by the use of variable rate speech encoding. The method and apparatus of a particularly useful variable rate speech encoder is described in detail in U.S. Pat. No. 5,414,796, entitled xe2x80x9cVARIABLE RATE VOCODERxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
The use of a variable rate speech encoder provides for data frames of maximum speech data capacity when the speech encoder is providing speech data at a maximum rate. When the variable rate speech encoder is providing speech data at a less that maximum rate, there is excess capacity in the transmission frames. A method for transmitting additional data in transmission frames of a fixed predetermined size, wherein the source of the data for the data frames is providing the data at a variable rate, is described in detail in U.S. Pat. No. 5,504,773, entitled xe2x80x9cMETHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSIONxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein. In the above mentioned patent application, a method and apparatus is disclosed for combining data of differing types from different sources in a data frame for transmission.
In frames containing less data than a predetermined capacity, power consumption may be lessened by transmission gating a transmission amplifier such that only parts of the frame containing data are transmitted. Furthermore, message collisions in a communication system may be reduced if the data is placed into frames in accordance with a predetermined pseudorandom process. A method and apparatus for gating the transmission and for positioning the data in the frames is disclosed in U.S. Pat. No. 5,659,569, entitled xe2x80x9cDATA BURST RANDOMIZERxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
A useful method of power control of a mobile in a communication system is to monitor the power of the received signal from the wireless communication device at a base station. In response to the monitored power level, the base station transmits power control bits to the wireless communication device at regular intervals. A method and apparatus for controlling transmission power in this fashion is disclosed in U.S. Pat. No. 5,056,109, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
In a communication system that provides data using a QPSK modulation format, very useful information can be obtained by taking the cross product of the I and Q components of the QPSK signal. By knowing the relative phases of the two components, one can determine roughly the velocity of the wireless communication device in relation to the base station. A description of a circuit for determining the cross product of the I and Q components in a QPSK modulation communication system is disclosed in U.S. Pat. No. 5,506,865, entitled xe2x80x9cPILOT CARRIER DOT PRODUCT CIRCUITxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
There has been an increasing demand for wireless communications systems to be able to transmit digital information at high rates. One method for sending high rate digital data from a wireless communication device to a central base station is to allow the wireless communication device to send the data using spread spectrum techniques of CDMA. One method that is proposed is to allow the wireless communication device to transmit its information using a small set of orthogonal channels. Such a method is described in detail in co-pending U.S. Pat. No. 6,396,804, entitled xe2x80x9cHIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
In the just-mentioned application, a system is disclosed in which a pilot signal is transmitted on the reverse link (the link from the wireless communication device to the base station) to enable coherent demodulation of the reverse link signal at the base station. Using the pilot signal data, coherent processing can be performed at the base station by determining and removing the phase offset of the reverse link signal. Also, the pilot data can be used to optimally weigh multipath signals received with different time delays before being combined in a rake receiver. Once the phase offset is removed, and the multipath signals properly weighted, the multipath signals can be combined to decrease the power at which the reverse link signal must be received for proper processing. This decrease in the required receive power allows greater transmission rates to be processed successfully, or conversely, the interference between a set of reverse link signals to be decreased.
While some additional transmit power is necessary for the transmission of the pilot signal, in the context of higher transmission rates the ratio of pilot signal power to the total reverse link signal power is substantially lower than that associated with lower data rate digital voice data transmission cellular systems. Thus, within a high data rate CDMA system, the Eb/N0 gains achieved by the use of a coherent reverse link outweigh the additional power necessary to transmit pilot data from each wireless communication device.
An additional benefit of the reverse link described in this co-pending application is that it generates less amplitude modulation (AM) interference due to its continuous-transmit nature. Thus, users with sensitive electronic equipment such as hearing aids and pacemakers will experience less interference than with a discontinuous transmit reverse link. Another example of the use of continuous transmission to reduce AM interference is given in co-pending U.S. Pat. No. 6,205,190, filed Apr. 29, 1996, entitled xe2x80x9cSYSTEM AND METHOD FOR REDUCING INTERFERENCE GENERATED BY A CDMA COMMUNICATIONS DEVICExe2x80x9d, assigned to the assignee of the present invention and incorporated herein by reference.
However, when the data rate is relatively low, a continuously-transmitted pilot signal on the reverse link contains more energy relative to the data signal. At these low rates, the benefits of coherent demodulation and reduced interference provided by a continuously-transmitted reverse link pilot signal may be outweighed by the decrease in talk time and system capacity in some applications. A method and system is needed to provide flexibility in reverse link transmission format as needed to optimize these tradeoffs.
Further, a communications device may need to go into hard handoff from a first system to a second system. If discontinuous transmission is possible, the device may search for the second system during the periods of non-transmission, while maintaining contact with the first system during periods of transmission.
The present invention is a novel and improved method and system for communicating a frame of information according to both a continuous transmit format and a discontinuous transmit format. In one aspect of the present invention, a method is disclosed for transmitting frames of information. The method includes transmitting information continuously throughout the frame when in a continuous transmit mode and the frame is of a first data rate of a plurality of data rates; and transmitting the information discontinuously in the frame when in a discontinuous transmit mode and the frame is of the first data rate. Thus, the present invention contemplates transmitting one or more data rates in either a continuous transmit mode or a discontinuous transmit mode.
The method may further include transmitting the information continuously throughout the frame when the frame is of a second data rate of the plurality of data rates. Thus, the present invention contemplates continuous transmission only for certain data rates, and selection between continuous and discontinuous transmission for other data rates.
In one embodiment of the present invention, the first data rate corresponds to a first transmit power and the second data rate corresponds to a second transmit power, and the first transmit power is less than the second transmit power. In this embodiment, the method includes transmitting the frame of the first data rate at the second transmit power when in the discontinuous transmit mode. Thus, frames transmitted in the discontinuous transmit mode may be transmitted at a higher transmit power than in the continuous transmit mode.
In one embodiment of the present invention, the information is transmitted at a fifty-percent duty cycle during the frame when in the discontinuous transmit mode. This may include transmitting the information during a second half of the frame.
Another embodiment of the present invention includes selecting between the continuous transmit mode and the discontinuous transmit mode in response to a transmit power of the wireless communication device. In other words, the present invention may include selecting the discontinuous transmit mode when the transmit power is less than a predetermined threshold. In an alternate embodiment, the present invention includes selecting between the continuous transmit mode and the discontinuous transmit mode according to a user-defined preference.
The present invention also contemplates a wireless communication device for transmitting frames of information. The wireless communication device includes a variable rate data source for generating the frames of information, each of the frames of information having one of a plurality of data rates. It also includes a transmitter for transmitting the information continuously throughout the frame when in a continuous transmit mode and when the frame is of a first data rate of the plurality of data rates, and for transmitting the information discontinuously in the frame when in a discontinuous transmit mode and when the frame is of the first data rate. Thus, the wireless communication device may transmit frames of a given data rate either continuously or discontinuously. A control processor selects between the continuous transmit mode and the discontinuous transmit mode. The wireless communication device may implement the method of the present invention as summarized briefly above.
The present invention also includes a method for receiving a frame of information in a wireless receiver, wherein the information may be continuously present throughout the frame or discontinuously present in the frame. This method includes filtering the frame of information in a sliding window filter to produce a sliding window phase estimate signal, filtering the frame of information in a block window filter to produce a block window phase estimate signal, and selecting between the sliding window phase estimate signal and the block window phase estimate signal in response to whether the information is continuously present in the frame.
In one embodiment of the present invention, the method includes selecting the sliding window phase estimate signal when the information is continuously present in the frame, and selecting the block window phase estimate signal when the information is discontinuously present in the frame. Additionally, the method may include selecting the block window phase estimate signal before and after a phase discontinuity in the frame.
The present invention further contemplates a wireless receiver for receiving a frame of information wherein the information may be continuously present throughout the frame or discontinuously present in the frame. The wireless receiver includes a sliding window phase estimator for filtering the frame of information in a sliding window to produce a sliding window phase estimate signal, a block window phase estimator for filtering the frame of information in a block window to produce a block window phase estimate signal, and a multiplexer for selecting between the sliding window phase estimate signal and the block window phase estimate signal in response to whether the information is continuously present in the frame. The wireless receiver may implement the method briefly described above.
Additionally, the present invention discloses a method, in a wireless communication system, for communicating a frame of information between a wireless communication device and a wireless base station in a continuous transmit mode and a discontinuous transmit mode. The method includes transmitting, from the wireless communication device, the information continuously throughout the frame when in the continuous transmit mode, and transmitting, from the wireless communication device, a first message notifying the wireless base station of an intention to transmit in a discontinuous mode. In response, the base station transmits a second message acknowledging the intention to transmit in the discontinuous mode, and the wireless communication device transmits the information discontinuously in the frame when in the discontinuous transmit mode, and in response to the second message.
In one embodiment, the method further includes demodulating the frame of information according to a continuous transmit format when the information is continuously present throughout the frame, and demodulating, the frame of information according to a discontinuous transmit format when the information is discontinuously present in the frame.
The present invention further contemplates a wireless communication system for communicating a frame of information in a continuous transmit mode and a discontinuous transmit mode. The wireless communication system includes a wireless communication device and a wireless base station that implement the method described briefly above.
In a final aspect of the present invention, a method and apparatus are disclosed for facilitating hard handoff from a first system to a second system. The device searches for the second system during the periods of non-transmission, while maintaining contact with the first system during periods of transmission.
Gating is supported for rate sets 3, 4, 5 and 6. When a frame is gated, only the symbols within the second half of the frame are sent. This means that symbols 6144 through 12287, numbering from 0, are transmitted. During gating, the maximum frame rate is rate xc2xd.
Normally, the blocks are transmitted using continuous transmission, with the exception of the rate xe2x85x9 frame which is gated. The continuous transmission reduces the interference in the audio band. The rate xe2x85x9 frame is gated because it improves the reverse link capacity and the mobile station talk time relative to when continuous transmission is used.
However, rate set 3, 4, 5 and 6 may be commanded into a mode where only rate xe2x85x9, rate xc2xc, and rate xc2xd frames are transmitted and are transmitted using gated transmission. This mode is used to allow the mobile station time to retune its receiver and search for systems using frequencies and other technologies (e.g. AMPS and GSM).
During gating, the second half of the frame is transmitted for the following reasons. First, the gating needs to be either in the first half or the second half of the frame. If it were not, then the frame would not contain a contiguous 10 milliseconds for searching. Second, the transmitted portion of the frame needs to occur later in the frame in order to allow the mobile station time to estimate the difference between the measured and expected forward signal to noise ratio. Therefore, during gating, the second half of the frame is sent.
In addition, rate set 3, 4, 5 and 6 may be commanded into a mode where all frames are transmitted using continuous transmission. This mode is used by mobile stations that may need to further reduce audio band interference.
A mobile station commanded into gated mode for searching will be commanded to periodically gate N frames out of M frames on the forward link and reverse link simultaneously, starting at system time T. The values of N and M depend on the technology being searched and the number of channels being searched.